1. Technical Field of the Invention
The present invention relates to a line driver having a self adjustable output impedance and, in particular, to a transformer line driver.
2. Description of Related Art
Line drivers having a controlled output impedance are well known in the art. See, B. Nauta, et al., "Analog Video Line Driver with Adaptive Impedance Matching," ISSCC98, pp. 318-19, 1998. A simplified schematic of one such driver 10 is illustrated in FIG. 1A. The driver 10 (also referred to as a "buffer") comprises an operational amplifier 12 whose negative input terminal receives an input voltage Vin. The output terminal of the operational amplifier 12 is connected to the gates of two field effect transistors 14 and 16, where the illustrated "N" value is equal to the ratio of their respective drain currents. The sources of the field effect transistors 14 and 16 are connected to a reference voltage Vdd. The drains of the field effect transistors 14 and 16 are connected to each other by a resistor (R1) 18. The drain of the field effect transistor 14 is connected in a feedback fashion to the positive input terminal of the operational amplifier 12, and is also connected to ground through a resistor (R2) 20. An output voltage Vout is supplied from the drain of the field effect transistor 16 to drive a transmission line 22 having a characteristic resistance equal to the load resistance (RL) 24. By properly selecting the values of the resistors R1 and R2 for the driver 10 in a well known manner (and as illustrated) with respect to the "N" value and the value of the load resistance RL, the value of the output impedance from the driver may be set (i.e., controlled) substantially equal to the load resistance RL. An advantage of this driver is its reduced power dissipation which makes it very attractive for implementation in an integrated circuit. However, with respect to an integrated circuit fabrication, the precise resistance values needed to achieve substantial matching of driver-line impedance are very difficult to consistently obtain.
It is recognized that it would be advantageous to be able to exercise some adjustment control over the output impedance of the driver following the setting of the resistance values. The driver of FIG. 1A may be modified, as shown in FIG. 1B, to provide for such an adjustment mechanism. Controllable source degeneration (through circuit 30) is applied to the transistors 14 and 16. The current ratio value "N" is electrically tunable (through circuit 30) via application of the voltage Vtune. In this implementation, the driver adapts to match the load resistance RL using a control loop 28 that integrates the output current of the transconductance amplifier onto the capacitor connected to Vtune for application to circuit 30 resulting in an adjustment to the source current of transistor 14 and a change in the value of N. At low frequencies, the control loop 28 forces Vout to equal Vin, in which case the gain of the driver is one. By then setting the resistances R1 and R2 as discussed above, approximate matching of the output impedance to the load resistance RL is obtained, with the control loop 28 further refining the matching.
Most telecommunications devices utilize a transformer decoupling of the driver and the transmission line. Because transformer driver-line decoupling is utilized in the push-pull configuration, a direct current output signal related to the load resistance is not available to be integrated by the control loop 28 and produce the adjustment signal Vtune. Furthermore, if the transmission line is relatively long, its direct current resistance is substantially different from the characteristic impedance. In such situations, the precision of the impedance adjustment provided by the FIG. 1B circuit is not sufficient. Additionally, the FIG. 1A prior art driver has not, historically, been well suited for use in a push-pull B-class circuit as two such drivers are needed and they do not operate well together in push-pull. When one half of the push-pull circuit (i.e., one driver 10) generates some voltage in one half of the primary coil of the transformer, a flyback voltage appears in the other half of the primary coil. This flyback voltage penetrates to the input of the operational amplifier 12 of the other driver 10 through the feedback circuit connections and corrupts driver operation.
There accordingly exists a need for a line driver having a self-adjustable output impedance with reduced power dissipation and improved power efficiency for implementation in an integrated circuit.